The present invention relates to an industrial vehicle and an axle pivot angle detecting apparatus for an industrial vehicle having a pivotal axle.
In a typical industrial vehicle such as a forklift, an axle for supporting the rear wheels pivots relative to a body frame for stabilizing the body frame. However, if such a forklift is steered to change directions, lateral force generated by centrifugal force acts on the vehicle and tilts the forklift. The tilting may incline the body frame and thus destabilize the vehicle.
Japanese Unexamined Patent Publication No. 58-211903 describes a forklift that has a detecting apparatus for detecting centrifugal force. If the detected centrifugal force exceeds a predetermined value when the vehicle changes directions, a locking mechanism locks the axle to the body frame to minimize tilting of the body frame. The forklift thus turns in a stable manner.
Japanese Unexamined Patent Publication No. 58-167215 describes a forklift that has a detecting apparatus for detecting the weight of a load on forks and the height of the forks. If the detected load weight and the fork height exceed predetermined threshold values, a locking mechanism locks the axle.
Japanese Unexamined Patent Publication No. 9-315125, which was filed by the present assignee, describes an axle locking apparatus illustrated in FIG. 12. Rear wheels 91 are supported by a rear axle 92. The rear axle 92 is pivotally supported on a frame 93 by a center pin 94. A hydraulic damper 95 is located between the rear axle 92 and the frame 93. An electromagnetic valve 96 is located on the damper 95. A controller (not shown) excites or de-excites a solenoid of the valve 96 thereby locking or unlocking the damper 95. Specifically, the controller causes the damper 95 to lock the rear axle 92 when the fork height and the cargo weight are greater than predetermined threshold values. The damper 95 functions as a device for locking the rear axle 92 as well as a shock absorber. The use of the damper 95 thus reduces the number of the parts in the apparatus and simplifies the construction.
Since the threshold values are set only for the fork height and the cargo weight, the rear axle 92 may be locked while tilted relative to the frame 93. For example, the rear axle 92 may be locked even if one of the rear wheels 91 is on a step or a bump. When the forklift moves to a flat road surface thereafter, the raised rear wheel 91 loses contact with the road surface. That is, since the center of gravity is relatively close to the front wheels when the forklift carries a relatively heavy load at a relatively high position, the rear wheel 91 that was lifted by a bump remains raised. As a result, the vehicle is supported only by three wheels, or by the front wheels and the other rear wheel 91. This destabilizes the vehicle.
Further, unlocking the rear axle 92 while one of the rear wheels 91 is raised causes the raised wheel 91 to fall on the ground, which results in a great shock. This may cause cargo on a pallet carried by the forks to shift. Particularly, when the rear axle is locked, the fork height and the cargo weight are greater than the threshold values, that is, the carried cargo is heavy and is held at a high position. In this state, unlocking the rear axle 92 may shift the cargo and disrupt the operation.
The locking mechanism according to the Publication No. 58-167215 locks the axle by inserting two blocks between the body frame and the axle. However, the blocks cannot be inserted when the axle is pivoted since pivoting of the axle narrows the space between the axle and the body frame. In other words, when raised, the axle cannot be locked. Therefore, there is no subsequent shock caused by a falling wheel.